It is frequently necessary to detect the presence and even the concentration of a gas species. For example, the detection of even a small amount of hydrogen before the launch of a space vehicle portends a serious problem. The most common optical detection method, infrared absorption, cannot be used for hydrogen and other homonuclear diatomic molecules. For these and other molecules, detection of Raman scattered light is advantageous, owing to the inherent linearity of the signal and its independence from the background gas composition, pressure, and temperature. However, a problem inherent in Raman and other inefficient optical scattering methods is the need to have a strong illumination source and an efficient method of light collection.
Two methods of providing strong illumination are to use a powerful laser and/or to place the sample within the laser cavity. The former method requires a bulky, expensive laser. The latter design is inherently not stable or rugged, and requires constant active adjustment to maintain alignment. With the latter method, problems may be caused by dirty windows on the sample or reference chamber and refractive index gradients; the latter may prevent use for real-time monitoring of a flowing sample. In addition, if a problem occurs, the entire system must be replaced or dealt with as a unit due to the integral construction.